In conventional rotary cutting tools, the peripheral cutting edges are designed to be equally spaced in order to allow even loading on the tool body as a whole. For this reason, the flutes of conventional cutting tools are of the same size and shape which allows equal load distribution among the cutting edges.
In general, rotary cutting tools are designed with multiple flutes spaced symmetrically around the circumference of the tool where the flutes run along a partial length of the tool ending at the tool shank. The tool shank is the portion of the tool that is mounted in a machine tool and the fluted portion is the portion of the tool that engages the workpiece. The total number of flutes may vary, and the flutes may be formed to extend either parallel to the longitudinal rotational axis of the tool or more commonly to extend about the rotational axis as a helix. In a helical arrangement, the cutting edges defined by the flutes are each described by a “helix angle,” which is the angle formed by a line tangent to the helix and a line parallel to the rotational axis of the tool.
Conventional rotary cutting tools perform adequately at conventional speeds (RPM) and feeds, however, at speeds and feeds higher than conventional, which is desirable for productivity, considerable performance decay is experienced. This performance decay is directly attributable to the presence and magnitude of vibration, specifically resonant vibration, as cutting force increases. At increased speeds and/or feeds, conventional helical and straight-fluted tools induce resonance, whereby the action of the tool cutting a workpiece has a tendency to enhance potential oscillatory energy when the frequency of the oscillations matches the system's natural frequency of vibration (its resonant frequency) or a harmonic thereof. The occurrence of uncontrolled resonant vibration inevitably results in a condition commonly referred to as “chatter,” which results in poor tool performance both in terms of life expectancy and workpiece quality. This is an undesirable occurrence.
Several approaches to solving the problem of chatter attempt to minimize the occurrence and resultant effect of resonant frequency vibration. This is generally accomplished by creating an irregular form on or in the leading edge of the flutes, thereby interrupting the tendency of the system to create an uncontrolled oscillation. Additionally, these approaches may also include an asymmetrical arrangement of the flutes around the periphery (circumferential index) of the tool in order to further interrupt resonant frequency vibration. The ultimate goal of this activity is to prolong tool life by limiting the destructive characteristics of vibration at higher than conventional speeds and feeds. It is not desirable to alter rotational speeds of a machine tool and rate of advance speeds of a cutting tool as such steps may interfere with optimal productivity.